1. Field of the Invention
The present invention relates to an ultrasonic coupler to be arranged between an ultrasonic probe and a subject to be observed, the ultrasonic coupler being adapted for use in an ultrasonic diagnostic apparatus.
2. Description of the Related Art
Ultrasonic diagnostic apparatuses which produce tomographic images of human bodies using ultrasonic waves are widely used. Observation of a diagnostic site of a human body, such as a thyroid gland, a carotid and a mammary gland, using an ultrasonic diagnostic apparatus is described below, referring to the accompanying drawings.
FIG. 5 is a typical diagram showing the ultrasonic probe for transmitting and receiving ultrasonic waves, which probe is kept in direct contact with a surface of a human body. FIG. 6 is a typical diagram showing the ultrasonic coupler, which is an excellent ultrasonic transmitting material, arranged between the ultrasonic probe and the surface of the human body to be observed.
When the ultrasonic probe 10 directly contacts the body surface 12, as shown in FIG. 5, an ultrasonic beam 16 is converged to an internal position away from the body surface 12. Therefore, if a diagnostic site 18 located near the body surface 12 is observed while the ultrasonic probe 10 is kept in direct contact with the body surface 12, the resolving power of the ultrasonic diagnostic apparatus will deteriorate. To improve the resolving power in observation of a diagnostic site 18 near the body surface 12, an ultrasonic coupler 20 (see FIG. 6) is arranged between the ultrasonic probe 10 and the body surface 12, thus causing the ultrasonic beam 16 to converge at a position nearby the diagnostic site 18.
The ultrasonic coupler 20 is manufactured by using a non-hydrated (non-water-containing) gel material such as urethane rubber or silicone rubber, or a high-molecular weight, hydrated (water containing) gel material such a polyvinyl alcohol (hereafter referred to as "PVA"), polyvinyl pyrrolidone (hereafter referred to as "PVP") and polyethylene oxide (hereafter referred to as "PEO").
Generally, ultrasonic waves are substantially attenuated in the non-hydrated gel material (approximately 2dB/cm/MHz). Therefore, ultrasonic waves transmitted from the ultrasonic probe become attenuated when passing through the ultrasonic coupler made of a non-hydrated gel material. Accordingly, it is difficult to transmit ultrasonic waves into a subject to be observed, and ultrasonic waves reflected from the internal site of the subject to be observed also become attenuated when passing through this ultrasonic coupler. Consequently, there is a problem in that the S/N ratio is undesirably low when using an ultrasonic coupler made of non-hydrated gel material.
On the other hand, attenuation of ultrasonic waves in the high-molecular weight hydrated gel material is extremely small (approximately 0.1 dB/cm/MHz). Therefore, a high S/N ratio can be obtained by using an ultrasonic coupler made of the high-molecular weight, hydrated gel material.
An aqueous solution which contains PVA as the high-molecular weight, hydrated gel material has a characteristic that bridge formation among molecules proceeds and the aqueous solution is gelled when it is frozen at a temperature below the freezing point and defrosted at room temperature. This gelled material (PVA gel material) excels in tear strength and elasticity and provides excellent mechanical characteristics, which meet the ultrasonic coupler (refer to Japanese Patent Publication No. 47-12854). Therefore, various types of ultrasonic couplers, made of PVA gel materials, manufactured by various methods have been proposed.